Method of coating electric lamp envelopes



March 17, 1959 D. s. GUSTIN 2,878,136

METHOD OF COATING ELECTRIC LAMP ENVELOPES,

Filed March 50, 1956 IN VEN TOR.

DAN/6L 6! 6w Tl/V ega ' ATTORNEY United States Patent METHOD OF COATING ELECTRIC LAMP ENVELOPES Daniel S. Gustiu, Saugus, Mass, assignor to Sylvania Electric Products Inc, Salem, Mass., a corporation of Massachusetts Application March 30, 1956, Serial No. 575,111

1 Claim. (.Cl. 117*17) This invention relates to the manufacture of electric lamps and more particularly to the manufacture of incandescent electric lamps provided with a coating of lightdiffu'sing material on the inner wall thereof.

In the manufacture of incandescent electric lamps, several methods, well known to those skilled in the art, have been employed heretofore to provide the inner wall of the lamp envelope with a light-difiusing coating.

An object of this invention is to provide the inner wall of incandescent lamp envelopes with a light-diffusing coating by electrostatically precipitating the light-difiusingmedium thereon from a stream of air in which the medium is suspended.

There are certain essential requirements in the selection, preparation and application of the light-diffusing medium which must be met in order to obtain a satisfactory coating.

Initial elforts to electrostatically precipitate light-diffusing materials on the inner wall of incandescent electric lamp envelopes were unsatisfactory because of the tendency for the particles of the light-diffusing materials to stick to one another both during milling and during electrostatic precipitation, with the result that a coating of inferior quality was obtained. In addition, it was noted that the coating possessed poor adhesion qualities.

It has been found that the tendency of the particles of light-diffusing material to cake during the preparation of the material and during its application by electrostatic precipitation as well as the failure to adhere satisfactorily may be corrected by the addition to the light-diffusing material of a small amount of an inert additive preferably of sub-micron particle size. The additive employed should be one which does not materially absorb the visible light from the lamp; it should be one which is chemically inert with respect to the light-diffusing material employed; it should be one which is not adversely afiected at temperatures normally encountered in lamp preparation and operation; and it should not be appreciably hygroscopic. Silicon dioxide and aluminum oxide meeting these requirements give good results.

Satisfactory results may be obtained when the quantity of the additive is between about 1% and about 15% by weight of the light-diffusing material. When less than about 1% is used, there does not appear to be enough present to overcome the caking tendency and improve the adhesion. When amounts in excess of 15% are employed, the application of a coating of uniform thickness becomes more difiicult because of what appears to be an electrical insulating effect of the additive.

In addition to the requirements noted above, there are also certain essential requirements in the application of the light-diffusing medium to the lamp envelope which must be met, in order to obtain a satisfactory coating. In order to insure proper adhesion of the coating particles to the lamp envelope, the envelope should be heated to a temperature sufiicient to make the glass slightly conductive. The exact temperature depends in part on the type of glass and wall thickness of the envelope. Although the glass should be heated to a temperature sufiicient to give it the degree of conductivity desired in order that adhesion may be efiected, too much heat will increase the conductivity to a point where the electrostatic field is dissipated and poor electrostatic precipitation results.

Since the particles retain a slight residual charge after they are precipitated onto the lamp envelope, it may be desirable in some cases to remove this charge in order to improve the adhesion. This maybe accomplished by exposing the lamp envelope to a very humid atmosphere, such as a steam jet for example. This exposure must be of sufficient duration to insure complete wetting of the coating by the steam. In effecting this exposure to a humid atmosphere, care must be exercised to make sure that steaming is adequate to effect a substantial improvement in the adhesion and yet not excessive to the point of washing off the particles. After steaming, the coated lamp envelope should be thoroughly dried to insure the removal of all moisture which would tend to vaporize during lamp sealing and subsequent operation of the lamp. Alternatively, removal of the slight residual electrical charge may be effected by high temperature baking in stead of steaming and drying.

In the accompanying drawing, Figure l is a plan view of an apparatus with which the method of this invention may be employed, and Figure 2 is a side elevational view of the apparatus of Figurel.

The apparatus illustrated in the drawing comprises a rotatable bulb-supporting head 10 mounted on a shaft 12 which is driven by motor 14 through pulleys 16 and 18 and belt 2%. An insulating standard 22 on table 24 supports metal burner tube 26. Burner 28 is mounted on one end of tube 26 and the other end of tube 26 is connected to a supply of a combustible gaseous mixture through tubing 30. Burner 28 is electrically connected to a high voltage D. C. power supply through metal tube 26, metal plate 32 mounted on insulating standard 22, and wire 34, one end of which is attached to plate 32 and the other end of which is connected to the power supply (not shown).

The head 10 is provided with O-rings 36 which retain bulb 38 in position on the head. A stationary feeding tube 4t supported by collar 42, extends through the head 10 and into bulb 38. A wire electrode 44 is attached to the upper end of feeding tube 40 and serves asthe ground electrode. The lower end of feeding tube 40 is connected, by tubing 46, to an apparatus for furnishing a supply of powder particles suspended in air, such as the smoke generator shown and described in the copending application of A. H. Nimblett, Jr., Serial No. 566,540, filed February 20, 1956, for example.

Examples of compositions of light-diffusing mediums which have been employed successfully will now be described. About 16 pounds of fine particle size wollastonite of a high degree of purity was mixed with about 1 pound of super-fine alumina (sold under the trade name of Alon C) in a blender for about 25 minutes to efiect an intimate mixture of the two materials and to break up any large aggregates which may be present. This mixture was then placed in a smoke generator such as the one shown and described in the above-identified copending application of Nimblett, Jr., for example, and used to coat the inner wall of incandescent lamp bulbs. If colored lamps of various shades are desired, pigments in the appropriate amounts may be added to the powder mixture without departing from the spirit of the invention. Finely powdered bone ash (calcium phosphate) mixed with superfine alumina in substantially the same proportions as the wollastonite mentioned above has also been found to give satisfactory results. Similarly, superfine silica (sold under the trade name of Cabosil) may be employed as the inert additive and used with either wollastonite or bone ash.

In practicing the method {6f this invention, a lamp bulb is first heated to a temperature between 225 F. to 275 F. and air is circulated through the heated bulb to sweep out any dirt which may be present and to remove any moisture which may be liberated by the heat. The cleaned bulb, heated sufficiently to make it slightly con ductive, is now ready for coating. Powder mixtures of the type described above are then precipitated electrostatically onto the inner wall of the lamp envelope in the manner described above.

The air stream carrying the powder from the smoke generator is introduced into the bulb 38 through tubing 46 and feeding tube 40, and the bulb-supporting head 10 is rotated, at about 50 R. P. M. for example, by the motor .14 through pulley 16, belt 20, pulley 18 and shaft 12. During their travel from the smoke generator, the powder particles become charged by frictional electrification and thus have a charge of static electricity when they enter the feeding tube 40. Due to this static charge they are well dispersed in the air stream essentially as individual particles. Electricalcontact to the outer surface of the bulb is established by utilizing the conductive property of a gas fiame playing on the bulb, the flame emanating from burner 28 which is connected to a high voltage D. C.

power supply (not shown) through burner tube 26, plate 32 and wire 34. The connection of this power supply between the gas burner 28 and the wire electrode 44 provides a strong electrostatic field between the bulb wall and the electrode .44 near the center of the bulb. The

charged particlesof powder leaving the feeding tube 40 move under the influence of the electric field, to the wall 4 that in order to obtain uniform condensation of the steam, the bulb 'wall temperature must be equalized. Thus, after the powder particles have been electrostatically precipitated onto the inner wall of the bulb, the bulb is cooled to reduce and equalize the bulb wall tem' perature and then steam is introduced into the bulb. Exposure of the coating to the steam should be sufiicient to insure. adequate distribution of the steam to wet all parts of the coating and, of course, care should be exercised to avoid excessive condensation and washing away of some of the coating. After steaming, the coated bulb is heated to bake out and remove the moisture introduced during steaming.

What I claim is:

The method of providing the inner wall of an electric lamp envelope with a light-diffusing coating which comprises electrostatically precipitating on the inner wall of a lamp envelope particles of a mechanical powdered mixture of a material selected from the group consisting of .wollastonite and bone ash, and a material selected from the group consisting of silicon dioxide being between about 1% and about 15% by weight of the said first mentioned material and aluminum oxide and having an average particle size below a micron.

References Cited in the file of this patent UNITED STATES PATENTS 1,830,165 Gustin' Nov. 3, 1931 2,438,561 Kearsley Mar. 30, 1948 2,538,562 Gustin et a1. Jan. 16,1951

- 2,545,896 Pipkin Mar. 20, 1951 2,601,124 Moulton June 17, .1952 2,661,438 Shand Dec. 1,-1953 2,682,478 Howse June 29; 1954 .1 FOREIGN PATENTS 686,101 Great Britain Jan. 21,1953

OTHER REFERENCES Ambert: Properties of Wollastonite and Diopsede with Admixtures of TiO ZrO and SiO;, Journal, American Ceramic Society, volume 28, 137-141 (1945).

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,878,136 March 17, 1959 Daniel So Gustin It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, lines 21, 22 and 23, strike out being between about 1% and about 15% by Weight of the said first mentioned material"; line 223, after "oxide" insert being between about 1% and about 15% by Weight of the said first=mentioned material Signed and sealed this 29th day of September 1959 (SEAL) Attest:

KARL Ho AXLINE ROBERT C. WATSON Commissioner of Patents Attesting Oificer 

